Export 2D Camera Motion as a vector for Vector Blur Node
Here is what I did to get it to work. (I didn't add an explanation to my comment, because I essentially just did, what you described in your question.) Anyways, here it goes:

I'm using Blender 2.79.

My Scene only contains a camera and an image plane, which I added via "Images as Planes". Then, I added keyframes to the Camera Rotation. I modified X-, Y-, Z-Rotation with a Noise Modifier.





I turned off Motion Blur in the Render Tab



And here are the Render Passes I used:



Then, turn on Compositing under Post Processing.

This is my Node Setup to add the Vector Blur:



Which gives me this result:



Note, that the image plane is NOT moving.

Have a look at the blend file:



EDIT:

A little side-note, that I've just noticed:

When I look at the vector pass (by Ctrl + Clicking with Node Wrangler Addon) I only sometimes see the actual colors in the viewer. Other times the viewer shows nothing. But this seems to be only an issue with displaying it, because, when I left-click on the viewer, I actually get a values at the cursor position. This can also be confirmed by saving it out. - The I get this picture, even though the viewer shows nothing: